On Wed, Jan 27, 2010 at 03:12:49PM +0100, Bernd Petrovitsch wrote:
> On Mit, 2010-01-27 at 13:30 +0100, Julia Lawall wrote:
> > On Wed, 27 Jan 2010, Bernd Petrovitsch wrote:
> > > On Mit, 2010-01-27 at 11:57 +0100, Julia Lawall wrote:
> > > > On Wed, 27 Jan 2010, Dan Carpenter wrote:
> > > >
> > > > > Fixing the places which assign negative values to unsigned variables is a good janitor task.
> > > >
> > > > I had the impression that assignment to -1 was done sometimes as a
> > > > portable way to initialize the variable to 0xffff (for any number of f's).
> Hmm, perhaps some experienced language lawyer can comment on the
> "portable".
Doesn't take a lawyer; conversion to unsigned types *IS* portable and defined
as arithmetics modulo 2^{width}. In particular, for any unsigned type T you
are going to have the same results from (T)-1 and ~(T)0 (and (T)-1L, etc.).
The value being converted is interpreted as an integer (i.e. the element of
$\Bbb Z$) and then taken modulo 2^{width}, regardless of the type it had
come from. So -1 is just fine and will result in 0xff....f of the right
width.
It's conversion to signed that is a mess if the value you are converting isn't
already in range representable by the type you are converting to.
Whether a specific example of conversion of negative to unsigned is a good
idea stylistically is a different question, of course, but that should be
taken on case-to-case basis.
--
To unsubscribe from this list: send the line "unsubscribe kernel-janitors" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
